The present invention relates to a lithium cobalt-based oxide having a layered structure, for use as a cathode material in a rechargeable lithium-ion battery. The oxide has a core-shell configuration and is provided with doping elements, oxides and a dedicated coating layer.
Lithium cobalt oxide has served as an archetypal cathode material for secondary Li batteries ever since the discovery by Mizushima et al. in 1980 of its electrochemical properties. Lithium cobalt oxide-based materials have a layered structure alternating CoO2 and LiO2-slabs of edge-shared CoO6 and LiO6 octahedra along the 001 direction of the hexagonal unit cell (space group R-3m). Such a layered structure is ideally suited to reversibly accommodate lithium by de-intercalation and intercalation during battery charge and discharge, respectively. Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. As today's consumer electronics demand rechargeable batteries with higher energy density, there is a surge towards LiCoO2-based materials with increased energy density for more demanding end applications.
The energy density (in Wh/L) of LiCoO2-based cathode materials is defined as the product of the average voltage during cycling (in V), the specific capacity (in mAh/g) and the gravimetric density (in g/cm3). Effective approaches to improve the energy density include:
(a) increasing the packing density, which usually requires to increase the particle size of the powder particles, and
(b) increasing the specific capacity by increasing the charge voltage. In a commercial cell, LiCoO2 is usually cycled with an upper cutoff voltage of about 4.35 V with respect to a graphite anode, and gives a specific capacity of 164 mAh/g. To obtain a higher capacity from LiCoO2, one must charge it to a potential above 4.35 V; typically 4.40V with a specific capacity of 172 mAh/g, and even up to 4.45V with a specific capacity of 182 mAh/g. However, repeated charge-discharge cycles using a higher upper cutoff voltage results in rapid capacity losses, thought to be caused by the structural instability of de-intercalated LiCoO2, and by the increase in side reactions with electrolyte.
The industrial applicability of these two approaches is thus limited by side problems. The latter route is limited by the unstable behavior of the charged electrode materials in contact with electrolyte at higher voltage. As lithium is removed from LixCoO2 (with x<1), changes in the electronic structure of LixCoO2 occur, characterized by a strong electron charge transfer to the cobalt and oxygen, resulting in a strong electronic delocalization. Charged LixCoO2 is a very strong oxidizer and has a highly reactive surface. The electrolyte is thermodynamically unstable in contact with such an oxidizing surface. A reaction with electrolyte, being the reducing agent, is strongly preferred energetically. Even at low temperature during normal cycling of a LiCoO2 cathode at high voltage, this parasite reaction slowly but continuously proceeds. Reaction products cover the surface and electrolyte is decomposed, and both effects continuously result in a deterioration of the electrochemical performance of the battery whereby a loss of capacity and a strong increase of resistance (also known as polarization) is observed.
In addition, in “Solid State Ionic, 83, 167 (1996)”, severe cobalt elution is reported for charged LixCoO2. The cobalt dissolution increases exponentially when the charge potential increases from 4.1V up to 4.5V in a coincell with respect to the lithium anode, and cobalt is deposited onto the negative electrode in cobalt metal form. A direct correlation between the coincell capacity fade and cobalt dissolution was established in this voltage range. In “Journal of Materials Chemistry, 21 (2011) 17754-17759”, Amine et al. emphasize that the dissolution of transition metal ions from the cathodes in Li-ion batteries is a detrimental phenomenon because these metal ions migrate from the cathode to the anode and are reduced to a metallic state onto the anode surface electrolyte interface (SEI). The metal or metal alloys deposited on the surface of the graphite anode has a negative effect on the stability of the SEI. Thus, this metal dissolution from the cathode and deposition onto the anode results in poor safety, poor cycling stability at higher voltage, and in poor storage properties of the charged cathode at elevated temperature. “J. Electrochem. Soc., 155, 711, (2008)” describes a mechanism where stray HF contained in electrolyte attacks LiCoO2 and causes Co-dissolution.
A first conclusion of this prior art study is that achieving a lithium cobalt oxide with a functional surface preventing side reactions and metal dissolution is desirable. On the other hand, increasing the particle size to increase the packing density impairs the power capabilities of rechargeable batteries. In order to meet the power requirements, the battery as a whole and particularly the active cathode material itself must have a sufficient high rate performance. Increasing the mean particle size reduces the effective solid-state lithium diffusion kinetics which eventually results in lowered rate performance.
A careful study of published results on cathode materials allows a better understanding of the limitations of LiCoO2 based rechargeable batteries. A fundamental limitation of state of the art LiCoO2-based materials lies in the Li-excess and particle size dilemma. This dilemma is recalled in detail in Yoshio, M. et al. (2009). Lithium-Ion batteries. New York: Springer Science+Business Media LLC.: the higher the corresponding lithium excess, expressed as the molar ratio Li:Co>>1.00—typically Li:Co is around 1.05—used for the synthesis; the higher the packing density and the higher the particle size; the lower the specific surface area (or BET) and the higher the base content and the lower the electrochemical power properties. The mechanism behind LiCoO2 particle growth and densification with increasing Li:Co is based on a so-called “lithium-flux effect” where the Li-excess acts as a flux agent enhancing the growth of LiCoO2 particles which eventually increases the packing density. Such LiCoO2 materials with dense and monolithic particle morphology are desirable to decrease the specific surface area and decrease the side-reaction area of the electrolyte with the cathode materials.
In W02010-139404, the authors illustrate the relationship between packing density, mean particle size and lithium excess used for the preparation of state of the art Mg and Ti doped LiCoO2. Typical packing densities of ca. 3.70 g/cm3 are achieved for 18 μm particles. The authors emphasize that large pressed densities are preferable and obtained with monolithic, potato-shaped and non-agglomerated primary LiCoO2 particles. However, the use of large Li:Co excesses to achieve larger monolithic particles results in poor electrochemical performances, with lower power (lower C-rate) and lower discharge capacity, which in return cancels the energy density gains achieved by increasing the particle size. These limitations in power and discharge capacity are resulting from two main factors: (i) a basic solid state diffusion limitation following a quasi-Fick-law (D˜vL2/t) as the particle size increases and (ii) introduction of structural defects with increasing Li-excess; for example as discussed by Levasseur in Chem. Mater., 2002, 14, 3584-3590 where the Li-excess is associated with Co substitution for Li in the CoO2 layers and with oxygen deficiency Li1+xCo1−xO2−x; further limiting the Li-ion diffusion mean free path within the particle. Last, such large Li:Co values also increase the pH, the free (or soluble) base content and the carbon content, which impairs safety, storage and bulging properties of charged cathodes.
In the prior art several approaches have been suggested to cope with these problems. They are usually associated with doping, coating or surface modification of LiCoO2 based materials. Co substitution for elements such as Mg, Ti and Al and improvement of high voltage properties is for example described in EP1137598B1. Such doping prevents the structural collapse and improves the cycle stability and reversible capacity of LiCoO2-based materials at high voltage. Industrial applicability is however limited as sintering of LiCoO2 is prevented by Ti-doping and hence requires higher sintering temperature or larger Li-excess. To achieve high voltage stability, LiCoO2 materials are usually coated (for example with Al2O3) or otherwise chemically modified (e.g. by providing a fluorinated surface). A problem is that coated dense LiCoO2 particles often have larger polarization and lower Li-ion diffusion resulting in lower reversible capacity and poor rate performance, so that a part of the gain of energy density achieved by charging to higher voltage is annulled by a lower intrinsic capacity.
As a consequence, the current state of the art synthesis does not allow to achieve dense, monolithic LiCoO2-based particles with superior energy density; electrochemical power and cycle life properties. Partial improvements and optimizations have been achieved, but the above mentioned basic problems have not yet been fully resolved. Hence there is clearly a need for high energy density LiCoO2 based cathodes which can be cycled in a stable manner, namely by reducing side reactions and metal dissolution in real cells at higher voltages. It is an object of the present invention to define a cathode material for high end secondary battery applications having a high packing density, high rate performance, improved discharge capacity and showing high stability during extended cycling at high charge voltage.